Kernel: Remove unnecessary includes from Thread.h

...and deal with the fallout by adding missing includes everywhere.

Kernel/Process.cpp

@@ -29,11 +29,13 @@
 #include <Kernel/PerformanceEventBuffer.h>
 #include <Kernel/PerformanceManager.h>
 #include <Kernel/Process.h>
+#include <Kernel/Scheduler.h>
 #include <Kernel/Sections.h>
 #include <Kernel/StdLib.h>
 #include <Kernel/TTY/TTY.h>
 #include <Kernel/Thread.h>
 #include <Kernel/ThreadTracer.h>
+#include <Kernel/TimerQueue.h>
 #include <LibC/limits.h>
 
 namespace Kernel {